fly in the ointment
fly in the ointment {n. phr.}, {informal}
An unpleasant part of a pleasant thing; something small that spoils your fun.
We had a lot of fun at the beach; the only fly in the ointment was George's cutting his foot on a piece of glass.
Your new job sounds too good to be true — interesting work, high pay, short hours. Isn't there any fly in the ointment?
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Source: A Dictionary of American Idioms
